This invention relates to an automatic transmission, particularly to a line perssure cotrol device provided with a hydraulic control circuit having cutback means.
Generally, an automatic transmission includes a torque converter, a tranmission gear mechanism having a planetary gear unit and friction coupling members for shifting the gear ratio, and a hydraulic control circuit for controlling hydraulic pressure to the friction coupling members. The friction coupling members are clutches, brakes, and the likes. The hydraulic control circuit includes pressure regulator means for regulating the hydraulic oil supplied from an oil pump to an appropriate line pressure, and shift valves for controlling the applying and releasing of coupling pressure to the respective friction coupling members.
In such an automatic transmission, the line pressure has been required to control in accordance with transmitted torque in order to develop the coupling pressure necessary for the transmitted torque, and assure a reduced driving loss of the oil pump and an improved fuel consumption. In view thereof, there has been proposed automatic tranmission provided with a hydraulic control circuit having cutback means. In such automatic transmissions, in addition to usual control of controlling the line pressure in accordance with the throttle opening of the engine, the so-called cutback control is executed in a particular gear ratio. In the cutback control, specifically, the line pressure is reduced when the transmission is set in a particular gear ratio. For example, Japanese Examined Patent Publication No. 46-33050 discloses an automatic transmission having a control valve responsive to hydraulic pressures in shifted gear ratios for reducing the line pressure stepwise as the transmission is progressively set in higher speed gear ratios. In other words, the control valve is capable of executing cutback control.
In the above-mentioned automatic transmission, when the transmission is set in a low speed gear ratio to transmit a great torque, the cutback control is not executed to maintain a high line presssure to develop a necessary coupling pressure. When the transmission is set in a high speed gear ratio to transmit a small torque, the cutback control is executed to assure a reduced driving loss of the oil pump.
However, this control, in which the cutback control is executed at all times when the transmission is set in a high speed gear ratio, gives bad influence to the cooling performance of an oil cooler. Specifically, the temperature of the hydraulic oil supplied to the friction coupling members and the torque converter is likely to rise is running time. The rise of temperature of the hydraulic oil impairs the reliabiltiy of the friction coupling members. For this reason, conventional automatic transmissions are provided with an oil cooler for cooling the hydraulic oil. However, when the cutback control is executed, a smaller amount of hydraulic oil is supplied to the oil cooler because of the fact that the line pressure is reduced. Consequently, the oil cooler cannot be so sufficiently performed as to keep the hydraulic oil at a lower temperature than a predetermined value, which thus lowers the reliability of the friction coupling members.